Emulsion polymers are typically stabilized by surfactants, or by a combination of surfactants and protective colloids such as polyvinylalcohol, hydroxyethyl cellulose (HEC), dextrin, and the like. Protective colloids have typically been used in the polymerization of highly reactive, hydrophilic monomers such as vinyl acetate, and common polymers that use this approach include for example polyvinylacetate, vinyl acetate-ethylene (VAE), and vinyl acrylics. One possible reason why the use of protective colloids is rare in the emulsion polymerization of hydrophobic monomers, such as styrene and butadiene, could be due to the fact that it is extremely difficult to make stable emulsions in these systems, especially if one targets a solids content of greater than 50 weight percent. It is interesting to consider the use of protective colloids such as polyvinylalcohol or HEC because of the unique rheology and tack properties these systems possess compared to conventional surfactant stabilized latices. Accordingly, the protective colloids are potentially very useful in many adhesive and coating-related applications.
While numerous patents and published articles exist illustrating the use of polyvinylalcohol or other colloids in vinyl acetate based polymers, a possible way of addressing the problem of coagulation in butadiene and styrene-based emulsions has only been recently reported. In particular, Kuraray Co. Ltd. in Yuki et al., Polymer International 30(4):512 (1993) proposes a mercapto terminated polyvinylalcohol which is grafted onto a styrene polymer to form a stable emulsion. This is a two-step process in which the first step is to make the mercapto functional polyvinylalcohol (Sato et al., Makromolekulare Chemie 194:175 (1993)). The functionalized polyvinylalcohol is then used in a conventional emulsion polymerization process to make a colloidally stabilized polystyrene emulsion. The use of this approach for making butadiene emulsions has been reported in Japanese Patent Nos. 5059106; 6128443; 6179705; and 7070989.
Another approach to making butadiene emulsions stabilized by polyvinylalcohol has been proposed in U.S. Pat. No. 5,200,459. In this instance, butadiene polymer latices stabilized by polyvinylalcohol are prepared by the emulsion polymerization of butadiene with other monomers in the presence of a solvent such as methanol. The methanol or other solvent is subsequently removed from the latex by a stripping process to yield a solvent-free latex.
Potential drawbacks exist with both of the above approaches. With respect to the first approach, the use of a mercapto-functional alcohol is restrictive because the process has trouble working with conventional polyvinylalcohols. With respect to the second approach, the use of a solvent to facilitate stabilization poses potential problems relating to solvent handling, recovery, and recycling.